Up to 9 million people contract tuberculosis every year and 50 million people are presently infected with Mycobacterium tuberculosis resistant to both first-line drugs isoniazid and rifampicin (WHO, Fact sheet No. 104, March 2007). Ethionamide (2-ethylthioiso-nicotinamide, 2-ethylpyrimidine-4-carbothioamide), a structural analogue of isoniazid, is currently the last line of defence in the treatment of multi-drug-resistant tuberculosis (MDR-TB). During 35 years of its clinical use, ethionamide has fortunately elicited little cross-resistance with isoniazid as both prodrugs have to be activated by different mycobacterial enzymes to develop their antimicrobial activity. Yet, ethionamide continues to be prescribed at hepatotoxic doses as a consequence of EthR repressing ethA, the monooxigenase that catalyses activation of the prodrug ethionamide into an antimycobacterial nicotinamide adenine dinucleotide derivative. Up to a 1 g/day are required for an acceptable concentration in blood (Holdiness, M. R., Clin Pharmacokinet 9, 511-44 (1984)), which is associated with severe side-effects including neurotoxicity and fatal hepatotoxicity.